The present invention is in the field of electrical power supply control and, more particularly, control of electrical power supply systems that provide power for loads with dynamically varying resistive and inductive characteristics.
Some power supply systems are constructed to provide alternating current (AC) with a controlled magnitude to loads which may be series combinations of resistance and inductance. One example of such a power supply requirement may arise when supplying current to exciter windings of a machine such as a starter-generator of an aircraft. AC power at a controlled frequency (e.g., 400 Hertz [Hz.]) may be supplied to the exciter windings to provide main generator excitation when the machine is in an engine-starting mode. As rotational speed of the machine varies, a corresponding change in excitation level is required. Thus, current supplied to the exciter winding must vary as a function of speed of the machine. This varying current must be accurately controlled during speed changes.
But, as speed changes, impedance (resistance and inductance) of the exciter winding changes. Consequently, a power supply system for such a machine must be capable of accurately controlling a dynamically changing current requirement in conditions of dynamically varying impedance.
Prior-art control systems have been employed to address this current control problem. In a typical prior-art system feedback signals may be generated by sensing the current in the exciter winding. A direct current (DC) representation of the sensed alternating current may be produced by rectification and filtering. The DC representation may be then provided to a proportional-integral controller for control of magnitude of alternating current that is delivered to the exciter winding. Large filters may be required for production of the DC feedback signal. Consequently, the transient performance of current control is poor. With use of DC feedback signal, information related to AC current waveform may be lost and a resultant alternating current to the exciter windings may be correspondingly distorted.
These distortions may lead to poor performance of the machine. Poor transient control and poor THD control may lead to current oscillations in the machine. The machine may then experience torque ripple and power loss.
As can be seen, there is a need to provide a power control system which may provide accurately controlled AC power with dynamically varying magnitude to a load with dynamically varying impedance. In particular, there is a need to provide such AC power with minimal THD and transient distortions.